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The Belle II Experiment: Status and Prospects universe Article The Belle II Experiment: Status and Prospects Paolo Branchini † INFN Sezione di RomaTre Via Della Vasca Navale, 84, 00146 Roma, Italy; [email protected] † On behalf of the Belle II Collaboration. Received: 16 September 2018; Accepted: 29 September 2018; Published: 1 October2018 Abstract: The Belle II experiment is a substantial upgrade of the Belle detector and will operate at the SuperKEKBenergy-asymmetric e+e− collider. The accelerator has already successfully completed the first phase of commissioning in 2016. The first electron versus positron collisions in Belle II were delivered in April 2018. The design luminosity of SuperKEKB is 8 × 1035 cm−2 s−1, and the Belle II experiment aims to record 50 ab−1 of data, a factor of 50 more than the Belle experiment. This large dataset will be accumulated with low backgrounds and high trigger efficiencies in a clean e+e− environment. This contribution will review the detector upgrade, the achieved detector performance and the plans for the commissioning of Belle II. Keywords: flavor; dark matter 1. Introduction Even though the Standard Model (SM) is currently the best description of the subatomic world, it does not explain the complete picture. The theory incorporates only three out of the four fundamental forces, omitting gravity. Moreover, there are also important questions that it does not answer, such as the matter versus antimatter asymmetry in the number of quark and lepton generations. Many New Physics (NP) scenarios have been proposed. Experiments in high-energy physics search for NP using two complementary approaches. The first, at the energy frontier, is able to discover new particles directly produced in pp collisions (ATLAS, CMS). Sensitivity to this production depends on the cross-sections and recorded statistics. The second approach, at the intensity frontier, seeks to reveal new weak interactions in the flavor sector beyond the SM. Such interactions can occur if a new particle exists and appears in an intermediate state of rare processes. The Belle II experiment aims to discover such interactions. The advantages of the Belle II experiment at the SuperKEKB B-factory with respect to a hadron-collider experiment are: 1. full solid angle detector coverage; 2. the relatively clean environment of e+e− collisions w.r.t. the hadronic environment; 3. the possibility to reconstruct the final state completely. A relatively low background environment allows for excellent reconstruction of the final system with photons in a wide energy region from neutrals, such as p0, h and h0. Due to the low track multiplicity, we have high B, D and t reconstruction efficiencies. As a result, B factories are also charm and t factories. Since e+e− collisions produce a clean samples of B mesons from the initial known Y(4s) state, missing-mass analyses based on the energy-momentum conservation law can be performed. Belle II also exploits the detection of decay products of one of B meson to be tagged. All these possibilities make the Belle II experiment unique for performing NP measurements and important cross-checks for many deviations from SM measured at the LHCb experiment [1]. Universe 2018, 4, 101; doi:10.3390/universe4100101 www.mdpi.com/journal/universe Universe 2018, 4, 101 2 of 8 2. The SuperKEKB Design Concept The SuperKEKBaccelerator [2] is upgraded from KEKB [3], as shown in Figure1. The target instantaneous luminosity is 8 × 1035 cm−2 s−1, and it is higher than that of KEKB by a factor of 40. The beam energies for the High Energy Ring (HER) and the Low Energy Ring (LER) are 7 GeV and 4 GeV, respectively. The energy of the LER has been increased to obtain larger dynamic acceptance. The designed beam currents of the HER and the LER are 2.6 A and 3.6 A, respectively. Several upgrades were performed to achieve this performance. The most important one is the nano-beam collision scheme. The lower emittance and the smaller vertical b∗ in the Interaction Point (IP) are critical. A new lattice design has been applied to the HER, and a completely new ring was built for the LER to reach the low emittance. A pair of new superconductive final focusing magnets was designed and fabricated. The described setup is essential to squeeze the beams. To achieve this, quadrupole magnets, compensation solenoid magnets and correction magnets in the single cryostat have been installed. Moreover, new TiN-coated beam pipes and the antechamber structure were designed and constructed to reduce the photoelectron cloud in the LER. A new damping ring was built in the injection linac section to meet the requirement of smaller acceptance for the LER. Moreover, higher bunch current and shorter bunch length might cause hardware trouble due to higher heat load of the Higher Order Mode (HOM) loss. Therefore, the bunch-by-bunch luminosity in KEKB 1.2 × 1031 cm−2 s−1 was rather low compared to other accelerators. Due to the double ring design, around 1600 bunches could be stored in each ring, and the world highest luminosity could be achieved. The storage of more bunches up to 3600 is relatively straightforward to get higher luminosity. However, the Radio Frequency (RF) limits the number of bunches to about 5000, thus allowing only a factor of three should the KEKB collision scheme be used. Figure2 shows the beam size at the collision point for KEKB and SuperKEKB. The overlap region is rather large in KEKB even for a small crossing angle. The collision spot is much smaller in SuperKEKB due to the smaller horizontal beam size (lower emittance) and the larger crossing angle. Therefore, we can think of each bunch as many subdivided “non-interacting” bunches. In this way, the nano-beam collision scheme can be exploited to reach the highest possible bunch-by-bunch luminosity. Figure 1. Schematic view of the SuperKEKBaccelerator. Universe 2018, 4, 101 3 of 8 Figure 2. The vertical beam size at collision for both beams at KEKB (a) and SuperKEKB (b). 3. The Belle II Experiment Belle II [4,5] is a hermetic magnetic spectrometer and is a major upgrade of the Belle experiment that operates at the B-factory SuperKEKB, located at the KEKlaboratory in Tsukuba, Japan. The SuperKEKB facility is designed to collide electrons and positrons at center-of-massp energies in the region of the U resonances. Most of the data will be collected at the U(4s) resonance ( s = 10.58 GeV), which is just above the threshold for B-meson pair production. Hence, in the case of BB production, no additional fragmentation particles are produced. The accelerator is designed with asymmetric beam energies to provide a boost to the center-of-mass system and thereby allow for time-dependent CP violation measurements. The boost is slightly lower than that at KEKB, which is advantageous for analyses with neutrinos and missing energy in the final state, which require a good detector hermeticity. SuperKEKB has a design luminosity of 8 × 1035 cm−2 s−1, with the aim to collect 50 ab−1 of data in eight years. The first data-taking period for physics analyses started in April 2018, with luminosity lower than the designed one. In this particular running condition (called Phase 2), which serves mainly for machine commissioning and beam background studies, we have reached a peak luminosity of 5 × 1033 cm−2 s−1 and had collected about 500 pb−1 of data by the end of July 2018. A new data-taking period (Phase 3) will start in February 2019, and luminosity is expected to increase to the designed value, while the background is expected to be significantly higher. From Phase 3 on, we will collect data with the upgraded detector: Belle II. The modified Belle II detector includes several renovated subsystems (see Figure3). Figure 3. Overview of the Belle II detector. DEPFET, DEpleted P-channel Field Effect Transistor. Universe 2018, 4, 101 4 of 8 The new Vertex Detector (VXD) consists of two sub-detectors: a Pixel Vertex Detector (PXD) including two layers of pixelated sensors based on DEpleted P-channel Field Effect Transistor (DEPFET) technology and a double-sided Silicon strip Vertex Detector (SVD) with four layers of silicon strip sensors. A factor of two on the vertex resolution compared to the Belle vertex detector is obtained with this strategy. The central tracking system is a large-volume Central Drift Chamber (CDC), the CDC surrounding the VXD. To be able to operate at the high event rates, CDC now features smaller cells. A particle-identification system includes the Time-Of-Propagation (TOP) system in the barrel region which is a kind of Cherenkov detector, and Aerogel Ring Image CHerenkov (ARICH) detector in the forward region. In the TOP system, the time of propagation and the impact position of a Cherenkov photon are measured. In the ARICH detector, the number and the position of Cherenkov photons are detected. The Electromagnetic CaLorimeter (ECL), based on CsI(Tl) crystals, is used to detect photons and identify electrons. New calorimeter electronics have been implemented to decrease the large level of noise due to the machine background. The K-Long and Muon (KLM) detector, located outside the superconducting solenoid, has been equipped with layers of scintillator strips with silicon photomultipliers to be able to operate at significantly higher neutron fluxes. 4. Belle II Schedule The Belle II schedule consists of two main phases before full physics commissioning, which will start in February 2019. During Phase 1, the solenoid was not active, and no collisions took place. The Belle II detector was in a roll-out position, and a system of radiation detectors called BEAST II (Beam Exorcism for A Stable Belle II experiment) has been placed at the interaction region.
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